OCEANOGRAPHIC REDOX CHANGES ASSOCIATED WITH THE LATE SILURIAN LAU EVENT AS EVIDENCED FROM THE BROWNSPORT FORMATION, TENNESSEE

The late Silurian is characterized by global oceanographic and biotic changes related to the Lau carbon isotope excursion (CIE). The Lau CIE is well documented in areas such as Sweden, Ukraine, Australia, and North America. This excursion is one of the largest positive carbon isotope excursions in the Phanerozoic, +8‰ or greater in some localities. The associated Lau-Kozlowski extinction event (LKE), was initially defined by conodonts and graptolites, but has also been confirmed in many other prevalent marine fauna, and is the most severe and widespread documented biotic event in the Silurian. Importantly, the LKE is known for having anachronistic facies comparable to those associated with larger mass extinction events such as the Permo-Triassic. The causes and consequences of this extinction event are not well understood with most work hypothesizing changes in the global carbon cycle, redox conditions and/or eustatic sea level. Here we report new paired δ³⁴S_CAS and δ¹³C_carb analyses from the Brownsport Formation, central Tennessee. This data will help to elucidate the temporal variation of the global carbon cycle and the duration of such a perturbation. Generally, positive excursions in both the δ¹³C and δ³⁴S data would indicate that the carbon isotope excursion was caused by the global burial of organic matter and pyrite. An expansion of euxinia is required to have a dramatic δ³⁴S excursion and could have been a fundamental factor in the extinction event. A positive excursion in only the δ¹³C data could be indicative of either anoxia leading to the burial of organic matter or the weathering of a carbonate platform and thus not largely tied to ocean oxygenation like that of oceanic anoxic events. This can be further clarified by the use of the I/Ca proxy. Additionally, we will analyze the section for I/Ca to fingerprint local temporal changes dissolved O₂ concentrations. This study has identified the Lau CIE in a new roadcut in central Tennessee, with heaviest values (+4.7‰) from the “Bob” Member of the Brownsport Formation. For the first time we present new δ³⁴S and I/Ca data. This study begins to test whether the Lau CIE was the result of the increased burial of organic carbon as a result of anoxia/euxinia or the weathering of a subaerial carbonate platform as a result of eustatic sea level fall.